Light-sensitive material for facile and rapid obtaining of images with developing process of dry treatment by heat is conventionally known. Such heat developable light-sensitive material and its image forming method is described, for example, in Japanese Patent Examination Publicaiton patent Nos. 4421/1968 and 4927/1968, page 553-555 of "Fundamentals of photographic engineering" (published by Corona, 1879) and page 9-15 of "Research Disclosure", June, 1978 (RD-17029).
There are two kinds of heat developable light-sensitive materials: one for black and white images and the other for color images. In recent years, development of heat developable color light-sensitive material using a variety of coloring matter donors to obtain color image have been attempted.
Various heat developable color light-sensitive materials utilize a variety of methods. For example, a method to obtain color image by coloring matter transfer after releasing or forming a diffusible coloring matter by heat development (hereinafter called transfer method) needs an image receiver to receive transcribed image, but is superior at stability and clearness of images and facility and speediness of processing. The heat developable color light-sensitive material and image forming process of transfer method is described in specifications of Japanese Patent Publications Open Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 12431/1984, 159159/1984, 181345/1984, 229556/1984, 2950/1985, 52643/1986, 61158/1986, 61157/1986, 180550/1984, 132952/1986, and 139842/1986, as well as U.S. Pat. No. 4,595,652, No. 4,590,154 and No. 4,585,267.
These heat developable light-sensitive materials contain light-sensitive silver halide, organic silver salt and binder, and additionally contain color providing material in case of color light sensitive material. It is well known in the art that the silver halide emulsion is made highly sensitive by chemical sensitization. Sulfur sensitization, reducing sensitization, selenium sensitization and sensitization by precious metal such as gold are used individually or in combination as chemical sensitization.
Further, addition of azaindenes during chemical ripening or before coating of the silver halide emulsion to obtain high sensitivity is described in British patent No. 1,315,755 and Japanese Patent O.P.I Publication No. 77223/1976 and No. 140335/1985.
It was difficult, however, to satisfy the demand of photography of high sensitivity and lower fog when conventional silver halide emulsion was used for heat developable light-sensitive material. Accordingly, a strong need exists for sensitivity improvement of heat developable light-sensitive material containing light-sensitive silver halide. In particular, it is desired to obtain a higher sensitivity for the layer containing blue light-sensitive silver halide than those for other color sensitive layers for example, green-sensitive layer and red-sensitive layer to avoid optical color mixing between blue-sensitive layer and other layers.